Many conventionally pumped or "artificial lift" oil wells use a downhole reciprocating plunger type of pump. The reciprocating downhole pump assembly is relatively long and thin to avoid restricting oil flow up the well and is typically actuated by a longer and thinner "sucker rod" extending to the surface. The surface end of the sucker rod is pivotally attached to a rocking beam drive unit.
Beam units are typically driven by counterweighted flywheels connected to electric motors or internal combustion engines. The beam units occupy a relatively large surface area when compared to the diameter of the sucker rod, pump, or wellhead. Beam units are also very heavy for some applications, raising concerns such as containing heavy rotating mass and adequacy of supports.
For other pumped well applications, special downhole or subsurface motors (e.g., small diameter reciprocating hydraulic actuators supplied by tubing connected to surface sources of pressurized hydraulic fluid) are directly coupled to the subsurface pump. Subsurface motor geometries are constrained by the available room within the well diameter and production fluid flow requirements. Because of these constraints, subsurface pumps are more common on large diameter, deep wells.
Although surface rocking beam and subsurface hydraulic motor devices are common for many pumped well applications, these devices cannot be used economically for all pumped well applications. In certain applications, such as an offshore platform having multiple conventional diameter wells, have no room for large footprint beam units and subsurface hydraulic pumps would unduly restrict oil production from these diameter wells. These applications can use sucker rod actuated resource fluid pumps, but each rod is activated by a small footprint surface hydraulic actuator and a pumping unit instead of larger and heavier beam units. These surface hydraulic actuator and pumping units have been used successfully for many years.
Methods and instrumentation to evaluate the performance of both surface beam driven and subsurface motor driven pumped well systems have been developed. In a standard API method for evaluating sucker rod system performance, the production system is first shut down to install a load cell (a displacement transducer typically already exists) on the sucker rod and then restarted to produce a dynamometer card. The dynamometer card is a cyclic graph of cyclic sucker rod tensile forces and displacements (as measured at the polished rod atop the wellhead and extrapolated or normalized for various conditions such as downhole pump depth, sucker rod properties, beam unit characteristics, motor power, pump speed, and production fluid pressure and flow data). Dynamometer cards are compared and correlated to standardized API cards to evaluate system performance. Standardized API cards show various normal performance and performance problem indications. A microprocessor can be employed to calculate, normalize, compare, store, and otherwise process the data.
Conventional on-site performance analysis of subsurface motor driven units also relies primarily upon surface measurements, but the primary measurements are now hydraulic fluid pressure and flow. These measurements are derived from surface pressure and flow transducers and are similarly combined with other surface measurements (e.g., production fluid pressure and flow) and constants (e.g., fluid and sucker rod properties) to extrapolate and evaluate downhole pump and well performance. Evaluation again involves comparison of data (which may be normalized) to known performance indications. A microprocessor can again be employed to process the data.
Both surface beam and subsurface hydraulic motor well performance analysis methods primarily rely on surface measurements which are extrapolated to subsurface conditions. Although downhole measurements would avoid these extrapolations, downhole measurements are costly. Still further, downhole transducers are also susceptible to malfunction and errors which may be caused by high downhole pressures and temperatures.
However, when surface hydraulic actuators and pumping units are monitored by performance analysis methods relying primarily on surface measurements, problems have been experienced. A major problem with sucker rod (force/displacement) analysis is access to the sucker rod. The actuator may not allow installation of a load cell without a costly disassembly. A problem with both conventional sucker rod dynamometer (force/ displacement plots) cards and hydraulic fluid pressure/flow graphs is extraneous perturbations. The perturbations do not appear to be correlatable to known pump or well performance indications on either type of analysis.
None of the conventional approaches known to the inventor avoids perturbation problems when evaluating the performance of a well using a surface mounted hydraulic actuator and pumping unit connected by a sucker rod to a downhole pump. Standard comparisons to API plots also do not appear to be as cost effective or reliable a method for analyzing surface hydraulic actuator pumped well performance as for analyzing a beam driven unit.